2024 May/June – Paper 21

Question 1: Osmosis and Plasmolysis in Onion Cells

1(a) Experimental Design and Interpretation

1(a)(i) Identifying Variables and Observations (1 Mark)

• Independent Variable:
  • Solution Concentration of Sodium Chloride
    • Example: 7 nm within the suitable range of 5 to 10 nm.
    • Requirement: Unit (mol dm⁻³) must be shown.
• Dependent Variable:
  • Number of Cells Showing Plasmolysis

1(a)(ii) Concentrations of Sodium Chloride Solutions (1 Mark)

• Three Concentrations:
  • 0.10 mol dm⁻³: U2 (Least plasmolysis)
  • 0.50 mol dm⁻³: U3
  • 1.00 mol dm⁻³: U1 (Most plasmolysis)
• Key Points:
  • Ensure accurate identification and labeling of each concentration with correct units.

1(a)(iii) Drawing Cells Showing Plasmolysis (5 Marks)

• Drawing Requirements:
  1. Minimum Size and Clarity:
     • All lines must be sharp and continuous.
  2. Number of Cells:
     • Draw four whole cells, ensuring each cell touches at least two others.
  3. Cell Walls and Membranes:
     • Draw the cell wall as two lines around each cell.
     • Draw three lines where cells touch to represent shared cell walls.
  4. Correct Cell Shape:
     • Accurately depict the typical shape of onion cells (often circular or oval).
  5. Labeling:
     • Label at least one cell surface membrane.
• Marks Allocation:
  • Each correct element contributes to the total of 5 Marks.

1(a)(iv) Explaining Plasmolysis (3 Marks)

• Key Explanation Points:
  1. Water Potential Gradient:
     • Statement: Water potential is lower in solution U1 than in the onion cells.
  2. Water Movement:
     • Statement: Water moves out of the cells from high water potential to low water potential by osmosis.
  3. Plasmolysis Observation:
     • Statement: The cell membrane pulls away from the cell wall.
• Alternative Explanation:
  • If no plasmolysis is observed in part (a)(iii), explain that water potential is not lower or that osmosis did not occur.
• Marks Allocation:
  • Any two correct points: 2 Marks
  • All three correct points: 3 Marks

1(b) Data Analysis and Graphing

1(b)(i) Calculating Percentage Change in Mass (1 Mark)

• Formula: Percentage Change=(Final Mass−Initial MassInitial Mass)×100\text{Percentage Change} = \left(\frac{\text{Final Mass} – \text{Initial Mass}}{\text{Initial Mass}}\right) \times 100Percentage Change=(Initial MassFinal Mass−Initial Mass​)×100
• Example Calculation:
  • Initial Mass (M): 2.3 g
  • Final Mass (F): 2.4 g
  • Calculation: 2.4−2.32.3×100=4.35%\frac{2.4 – 2.3}{2.3} \times 100 = 4.35\%2.32.4−2.3​×100=4.35%
• Key Points:
  • Correctly apply the formula.
  • Ensure units are included.

1(b)(ii) Additional Calculations (1 Mark)

• Procedure:
  • Add values obtained from (b)(i) and (b)(ii), then divide by 3.
• Example:
  • If results from (b)(i) and (b)(ii) are 4.35% and 4.35%, then: 4.35+4.353=2.9%\frac{4.35 + 4.35}{3} = 2.9\%34.35+4.35​=2.9%
• Marks Allocation:
  • Correct calculation method: 1 Mark

1(b)(iii) Drawing and Labeling a Graph (4 Marks)

• Graph Requirements:
  1. Axes Labels:
     • X-axis: Sucrose concentration (mol dm⁻³)
     • Y-axis: Mean percentage change in mass
  2. Scales:
     • X-axis: 0.2 mol dm⁻³ to 2 mol dm⁻³, labeled at least every 2 mol dm⁻³.
     • Y-axis: 2% to 2 cm (assuming correct scale), labeled at least every 2%.
  3. Data Plotting:
     • Plot all five data points using small crosses or dots within circles.
  4. Line of Best Fit:
     • Connect points with a thin, ruled line of best fit.
• Marks Allocation:
  • Proper labeling and scaling: 2 Marks
  • Correct plotting and line of best fit: 2 Marks

1(b)(iv) Estimating Values from the Graph (1 Mark)

• Key Points:
  • Accurately estimate specific data points based on the graphed trend.
• Marks Allocation:
  • Correct estimation: 1 Mark

Question 2: Plant Root Anatomy and Starch Concentration

2(a) Drawing and Labeling Root Sections (5 Marks)

• Drawing Requirements:
  1. Utilization of Space:
     • Use most of the available drawing area.
  2. Correct Section:
     • Draw the correct section of the root without including individual cells.
  3. Number of Tissues:
     • Accurately represent the correct number of different tissues present.
  4. Proportion of Vascular Tissue:
     • Accurately depict the proportion of vascular tissue relative to the entire root section.
  5. Labeling:
     • Include labels for key structures, specifically labeling the xylem tissue.
• Marks Allocation:
  • Each correct element contributes to the total of 5 Marks.

2(b) Starch Concentration and Data Interpretation

2(b)(i) Setting Up Serial Dilutions (3 Marks)

• Procedure:
  1. Concentrations Used:
     • 0.1 mol dm⁻³, 0.01 mol dm⁻³, 0.001 mol dm⁻³, 0.0001 mol dm⁻³
     • Ensure % concentration is mentioned where required.
  2. Transfer Process:
     • Transfer 1 cm³ from the previous beaker to the next.
  3. Adding Water:
     • Add 9 cm³ of water to each beaker.
• Marks Allocation:
  • Correct concentrations and procedural steps: 3 Marks

2(b)(ii) Creating a Table of Starch Concentrations (3 Marks)

• Table Requirements:
  1. Headings:
     • Independent Variable: Percentage concentration of starch (no units in the table body).
     • Dependent Variable: [Specify as per experiment, e.g., percentage concentration of starch]
  2. Symbols:
     • Use symbols to represent concentrations.
     • Ensure symbols for higher concentrations (R1) are more numerous than those for lower concentrations (R2).
• Marks Allocation:
  • Correct headings and symbols: 3 Marks

2(b)(iii) Observing and Recording Color Changes (1 Mark)

• Key Points:
  • Use appropriate color indicators to represent starch presence.
  • Ensure color changes are accurately reflected in the observations.
• Marks Allocation:
  • Correct use of color: 1 Mark

2(b)(iv) Recording Observations (1 Mark)

• Key Points:
  • Record that R1 shows a higher number of symbols (indicating more starch) than R2.
• Marks Allocation:
  • Correct recording of observations: 1 Mark

2(b)(v) Estimating Starch Concentrations (1 Mark)

• Key Points:
  • Accurately estimate starch concentrations for R1 and R2 based on experimental results.
• Marks Allocation:
  • Correct estimation: 1 Mark

2(b)(vi) Suggesting Improvements to the Experiment (2 Marks)

• Suggestions:
  1. Repeat and Find the Mean:
     • Increase reliability by averaging multiple trials.
  2. Use a Colorimeter:
     • Enhance accuracy in measuring color changes quantitatively.
  3. Use More Concentrations with Narrower Intervals:
     • Improve precision in determining the relationship between concentration and starch presence.
• Marks Allocation:
  • Any two valid suggestions: 2 Marks

2(b)(vii) Interpreting Results (1 Mark)

• Key Points:
  • R2 has less starch present compared to R1.
• Marks Allocation:
  • Correct interpretation: 1 Mark

2(c) Comparing Root Structures (3 Marks)

• Differences to Highlight:
  1. Root Hairs:
     • Figure 2.4: Absent
     • Figure 2.5: Present
  2. Vascular Bundle Size:
     • Figure 2.4: Smaller
     • Figure 2.5: Larger
  3. Endodermis Thickness:
     • Figure 2.4: Thinner
     • Figure 2.5: Thicker
  4. Cortex Size:
     • Figure 2.4: Larger
     • Figure 2.5: Smaller
• Marks Allocation:
  • Highlighting three correct differences: 3 Marks

Question 3: Cardiovascular Structures and Gas Exchange

3(a) Comparing Arteries, Capillaries, and Veins (3 Marks)

• Table Comparison:FeatureArteryCapillaryVeinSmooth Muscle✓X✓Endothelium✓✓✓Tunica Media✓X✓
• Key Points:
  • Arteries and Veins:
    • Contain smooth muscle and tunica media.
    • Arteries: Thick walls for high-pressure blood flow.
    • Veins: Thinner walls but still contain smooth muscle.
  • Capillaries:
    • Lack smooth muscle and tunica media.
    • Composed solely of endothelium, facilitating gas exchange.
• Marks Allocation:
  • Correctly completing the table: 3 Marks

3(b) Gas Exchange in Lungs (5 Marks)

3(b)(i) Labeling Gas Movement Arrows (1 Mark)

• Oxygen (O₂):
  • Moves from the alveolar space into the capillary lumen.
• Carbon Dioxide (CO₂):
  • Moves from the capillary lumen into the alveolar space.
• Marks Allocation:
  • Correct labeling of gas movements: 1 Mark

3(b)(ii) Factors Enhancing Gas Exchange (4 Marks)

• Key Factors:
  1. Deoxygenated Blood:
     • Arrives in alveolar capillaries with low partial pressure of oxygen.
  2. Oxygen Uptake:
     • Oxygen binds to hemoglobin, forming oxyhemoglobin.
  3. Continuous Removal of Oxygenated Blood:
     • Transported away via pulmonary veins.
  4. Extensive Capillary Network:
     • Increases surface area for gas exchange.
  5. Ventilation:
     • Maintains a high concentration gradient between alveolar air and blood.
• Alternative Points:
  • Ventilation maintains a large concentration difference, enhancing diffusion.
• Marks Allocation:
  • Any four correct factors: 4 Marks

3(c) Identifying Pulmonary Vein (3 Marks)

• Characteristics:
  • Type of Vein: Pulmonary vein is a semilunar vein.
  • Other Semilunar Veins: Aortic and pulmonary veins.
  • Function: Carries oxygenated blood from the lungs to the heart.
• Marks Allocation:
  • Correct identification and labeling: 3 Marks

Question 4: Plant Physiology and Biochemical Processes

4(a) Suberin and Lignin in Plant Tissues (4 Marks)

• Suberin:
  • Function: Hydrophobic barrier found in the Casparian strip of endodermal cells.
  • Properties:
    • Impermeable to water, forcing water to move through the symplast.
    • Controls the movement of solutes into the xylem.
    • Prevents toxins and pathogens from entering the vascular system.
• Lignin:
  • Function: Provides structural support in cell walls of xylem vessel elements.
  • Properties:
    • Hydrophobic and impermeable to water loss.
    • Prevents collapse of xylem vessels under negative pressure during water transport.
    • Adds rigidity and strength to plant tissues.
• Marks Allocation:
  • Any four correct points covering suberin and lignin: 4 Marks

4(b) Enzyme Mechanism: Laccase Action on Monolignols (5 Marks)

• Laccase Enzyme:
  • Function: Catalyzes the oxidation of monolignols to form lignin.
• Mechanism:
  1. Induced Fit:
     • Active site undergoes a shape change to bind monolignols.
  2. Enzyme-Substrate Complex:
     • Monolignols bind to the active site, facilitating the reaction.
  3. Catalysis:
     • Lowers the activation energy required for the oxidation of monolignols.
  4. Product Release:
     • Laccase returns to its original shape after the reaction.
• Additional Points:
  • Cofactors: Requires copper ions for activity.
  • Hydrogen Bonds: Monolignols are held in place by temporary hydrogen bonds.
• Marks Allocation:
  • Any five correct points covering laccase action and enzyme mechanics: 5 Marks

Question 5: Signal Transduction and Cell Cycle Regulation

5(a) Ligand Binding and Signal Transduction (2 Marks)

• Process Overview:
  1. Ligand Secretion:
     • Cells secrete ligands or transport them through the circulatory system.
  2. Receptor Binding:
     • Ligands bind to specific, complementary receptors on target cells.
  3. Signal Transduction:
     • Binding triggers intracellular reactions, such as the activation of secondary messengers (e.g., cyclic AMP).
     • Initiates an enzyme cascade within the target cell.
• Key Points:
  • Specificity: Ligands must specifically bind to their receptors.
  • Amplification: Secondary messengers amplify the signal inside the cell.
• Marks Allocation:
  • Any two correct points describing ligand binding and signal transduction: 2 Marks

5(b) Non-Competitive Inhibition of Enzymes (2 Marks)

• Mechanism:
  1. Binding Site:
     • Non-competitive inhibitors bind to an allosteric site, not the active site.
  2. Conformational Change:
     • Binding induces a change in the enzyme’s shape.
  3. Effect on Active Site:
     • Active site becomes less complementary to the substrate, reducing enzyme activity.
• Key Points:
  • Irreversible: Non-competitive inhibition cannot be overcome by increasing substrate concentration.
  • Enzyme Functionality: Alters the enzyme’s ability to catalyze reactions without directly blocking substrate binding.
• Marks Allocation:
  • Any two correct points explaining non-competitive inhibition: 2 Marks

5(c) Cell Cycle Inhibitors (4 Marks)

5(c)(i) Palbociclib / p21Cip1 (2 Marks)

• Function:
  • Role in Cell Cycle: Inhibits cyclin-dependent kinases (CDKs) necessary for DNA replication.
  • Effect: Stops DNA replication during the S phase, preventing cell division.
• Key Points:
  • Role in Cancer Treatment: Halts uncontrolled cell proliferation by disrupting normal DNA replication.
• Marks Allocation:
  • Correct explanation of inhibitor function: 2 Marks

5(c)(ii) RO-3306 (2 Marks)

• Function:
  • Cell Cycle Phase: Stops the cell cycle in the G₂ phase.
  • Effect: Prevents mitosis, maintaining high numbers of mitochondria and ensuring DNA replication has occurred.
• Key Points:
  • Cell Cycle Control: Inhibits progression into mitosis, thereby controlling cell proliferation.
  • Research Applications: Used to study cell cycle dynamics.
• Marks Allocation:
  • Correct explanation of inhibitor function: 2 Marks

5(d) Control of the Cell Cycle (2 Marks)

• Mechanism:
  • Stopping Before Mitosis/Cytokinesis:
    • Prevents uncontrolled cell division, which can lead to tumor growth.
• Effect:
  • Tumor Suppression: Reduces the risk of increasing tumor size by regulating cell division.
• Marks Allocation:
  • Correct explanation of cell cycle control: 2 Marks

Question 6: Molecular Biology and Genetics

6(a) Differences Between mRNA and DNA

6(a)(i) Antigen-Binding Sites (2 Marks)

• Key Differences:
  1. Shape Complementarity:
     • Antigen-Binding Site (Variable Region): Has a shape that is complementary to the antigen; determined by the tertiary/quaternary structure.
  2. Amino Acid Sequence:
     • Specific sequences of amino acids in the binding site allow for specific binding.
  3. Flexibility:
     • Variable regions can adopt different conformations to accommodate different antigens.
• Marks Allocation:
  • Any two correct points: 2 Marks

6(a)(ii) Flexibility for Binding (1 Mark)

• Key Points:
  • Flexibility: Allows different angles or movement in the variable regions to bind various antigens.
• Marks Allocation:
  • Correct explanation of flexibility: 1 Mark

6(a)(iii) Opsonisation and Phagocytosis (1 Mark)

• Key Points:
  • Opsonisation: Antibodies bind to antigens on pathogens, marking them for engulfment by macrophages.
  • Facilitates Destruction: Enhances phagocytosis of the pathogen.
• Marks Allocation:
  • Correct explanation of opsonisation: 1 Mark

6(b) Alternative Splicing (2 Marks)

• Key Points:
  1. Removal of Introns:
     • Introns (non-coding regions) are removed from the primary transcript.
  2. Joining of Exons:
     • Exons (coding regions) are joined together in different sequences or combinations, leading to multiple mRNA variants from a single gene.
  3. Resulting Diversity:
     • Not all exons are used in every mRNA variant, allowing for the production of different proteins from the same gene.
• Marks Allocation:
  • Any two correct points: 2 Marks

General Exam Tips

• Diagram Labeling:
  • Ensure all diagrams are accurately labeled with key structures.
  • Include annotations where necessary to clarify functions and interactions.
  • Practice drawing clear and proportionate diagrams as per marking criteria.
• Units and Conversions:
  • Always include appropriate units in measurements (e.g., mol dm⁻³, nm).
  • Be comfortable converting between units (e.g., nm to µm).
• Understanding Processes:
  • Focus on understanding the mechanisms behind physiological and biochemical processes rather than just memorizing facts.
• Application of Knowledge:
  • Be prepared to apply concepts to different scenarios, such as how mutations affect protein function or how transport mechanisms respond to environmental changes.
• Answer Structuring:
  • Structure answers clearly, addressing each part of the question.
  • Use scientific terminology accurately and concisely.
• Time Management:
  • Allocate time based on the marks available for each question.
  • Ensure you answer all parts of multi-part questions.
• Practice Past Papers:
  • Familiarize yourself with the exam format and question styles.
  • Time yourself while practicing to improve speed and accuracy.
• Review Key Concepts:
  • Revisit topics such as osmosis, cell structure, plant physiology, signal transduction, cell cycle regulation, and molecular biology.
• Understand Marking Schemes:
  • Review the marking criteria to understand what examiners are looking for in your answers.
• Clarify Doubts:
  • If unsure about any topic, seek clarification from teachers or reliable study resources.